Experimental evaluation of spectral efficiency from a circular array antenna producing a Laguerre-Gauss mode

We present the four-dimensional volumetric electromagnetic field measurements (x, y, z and frequency) of the complex radiated field produced by an 8-element circular antenna array. The array is designed to produce a Laguerre-Gauss (LG) mode l = +1 over the frequency range of 9-10 GHz. We evaluate our findings in terms of far-field LG mode purity and spectral efficiency in terms of the quadrature amplitude modulation (QAM) modulation scheme that can be supported. The application of LG modes in radio systems is as a means of multiplexing several data streams onto the same frequency, polarization and time slot, thus making a highly spectrally efficient transmission system or enhancing radar systems by means of exploiting mode behaviour as an additional degree of freedom. Our results show that for the circular antenna array, we find that mode purity is sufficient to support binary phase shift keying or quadrature phase shift keying modulation over a 0.3 GHz bandwidth, which corresponds to a spectral efficiency of 1.5 b s^-1 Hz^-1 per mode. Closer to the antennas' design frequency, 256QAM modulation may be supported over a 0.05 GHz band, and which corresponds to a spectral efficiency of 11 b s^-1 Hz^-1 per mode. We anticipate the practical insights provided in this paper contribute to the successful design of such systems.